A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project small patterns onto the substrate, radiation is used having a wavelength which is within the so-called extreme ultraviolet (EUV) range which is electromagnetic radiation having a wavelength typically between 5 nm and 40 nm. In addition to EUV radiation, EUV radiation sources generate contaminant material that may be harmful for both the optics, the sensors configured to measure a quantity relating to the radiation and the working environment in which the lithographic process is carried out. Such is especially the case for EUV sources operating via a laser-induced plasma. Hence, in lithography in which EUV radiation is used for projecting patterns onto the substrate, a desire exists to limit the contamination of the optical system that is arranged to condition the beams of radiation coming from the EUV source, as well as the aforementioned sensors.
It has been proposed to arrange a screen to protect the sensor from incoming particles emitted from the source. However, such a screen may be positioned between the source and the sensor, thereby blocking the radiation from the sensor.
In order to allow radiation to reach the sensor it has been proposed that a mirror is provided, the mirror being configured to redirect extreme ultraviolet radiation emitted by the source and passing the screen, to the sensor.